Nitrogen-based atmospheres have been routinely used by the heat treating industry both in batch and continuous furnaces since the mid seventies. Because of low dew point and virtual absence of carbon dioxide and oxygen, nitrogen-based atmospheres do not exhibit oxidizing and decarburizing properties and are therefore suitable for a variety of heat treating operations. More specifically, a mixture of nitrogen and hydrogen has been extensively used for annealing low to high carbon and alloy steels as well as annealing of non-ferrous metals and alloys such as copper and gold. A mixture of nitrogen and a hydrocarbon such as methane or propane has gained wide acceptance for neutral hardening and decarburization-free annealing of medium to high carbon steels. A mixture of nitrogen and methanol has been developed and used for carburizing of low to medium carbon steels. Finally, a mixture of nitrogen, hydrogen, and moisture has been used for brazing metals, sintering metal and ceramic powders, and sealing glass to metals.
A major portion of nitrogen used by the heat treating industry has been produced by distillation of air in large cryogenic plants. The cryogenically produced nitrogen is generally very pure and expensive. To reduce the cost of nitrogen, several non-cryogenic air separation techniques such as adsorption and permeation have been recently developed and introduced in the market. The non-cryogenically produced nitrogen costs less to produce, however it contains from 0.2 to 5% residual oxygen, making a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen in continuous annealing and heat treating furnaces very difficult if not impossible for some applications. Several attempts have been made by researchers to substitute cryogenically produced nitrogen directly with that produced non-cryogenically but with limited success even with the use of an excess amount of a reducing gas. The problem has generally been related to severe surface oxidation of the heat treated parts both in the cooling and heating zones of the furnace, resulting in rusting and sealing. The use of non- cryogenically produced nitrogen has therefore been limited to applications where surface oxidation, rusting and sealing can be tolerated. For example, non-cryogenically produce nitrogen has been successfully used in oxide annealing of carbon steel parts which are generally machined after heat treatment. Its use has, however, not been successful for controlled oxide annealing of finished carbon steel parts due to the formation of scale and rust.
To exploit the cost advantage offered by non-cryogenically produced nitrogen over that produced cryogenically, researchers have been working on processes or methods to substitute non-cryogenically produced nitrogen for that produced cryogenically. For example, furnace atmospheres suitable for heat treating applications have been generated from non-cryogenically produced nitrogen by removing residual oxygen or converting it to an acceptable form in external units prior to feeding the atmospheres into the furnaces. Such atmosphere generation methods have been described in detail in French publication numbers 2,639,249 and 2,639,251 dated Nov. 24, 1988 and Australian patent application numbers AU45561/89 and AU45562/89 dated Nov. 24, 1988. The use of an external unit considerably increases the cost of non-cryogenically produced nitrogen for the user in controlled furnace atmosphere applications. Thus, industry has not adopted non-cryogenically produced nitrogen for these applications.
Researchers have also been experimenting with the addition of a number of reducing gases with non-cryogenically produced nitrogen into the hot zone of furnaces in attempts to produce atmospheres acceptable for heat treating ferrous and non-ferrous metals and alloys. For example, methanol has been added with non-cryogenically produced nitrogen in batch furnaces to successfully generate atmosphere suitable for carburizing carbon steels. This process has been described in detail in papers titled , "Carburizing with Membrane N.sub.2 : Process and Quality Issues", published in Heat Treating, pages 28-32, March 1988 (P. Murzyn and L. Flores, Jr.), "New Method of Generating Nitrogen for Controlled Atmosphere Heat Treatment at Torrington Shiloh Plant", published in Industrial Heating, pages 40-46, March 1986 (H. Walton), "The Use of Non-Cryogenically Produced Nitrogen in Furnace Atmospheres", published in Heat Treatment of Metals, pages 63-67, March 1989 (P. F. Stratton) and "How PSA Nitrogen Works in a Heat Treating Shop", published in Heat Treating, pages 30-33, November 1989 (D. J. Bowe and D. L. Fung). This process, as mentioned above, is suitable for carburizing carbon steels only in the batch furnaces. It has neither been tried nor used for carburizing parts in continuous furnaces. Furthermore, it has not been used successfully for annealing and heat treating parts made of ferrous and non-ferrous metals and alloys in continuous furnaces with separate heating and cooling zones.
Other reducing gas such as methane has been added into the hot zones of continuous furnaces with non-cryogenically produced nitrogen in attempts to generate atmospheres suitable for oxidation and decarburization-free annealing or hardening of carbon steels. The use of methane has, however, not been successful due to excessive oxidation and decarburization of the parts, as described in the paper by P. F. Stratton referred to above. The author concluded that the oxidation and decarburization problems were related to the slow rate of reaction between oxygen and methane at low temperatures and short residence times in the continuous furnaces used for oxide and decarburize-free annealing. The paper also concluded that non-cryogenically produced nitrogen would be cost competitive to cryogenically produced nitrogen only at residual oxygen levels below about 0.2%, if at all possible.
Hydrogen gas has also been tried as a reducing gas with non-cryogenically produced nitrogen for oxide-free annealing of carbon steels in a continuous furnace. Unfortunately, the process required large amounts of hydrogen, making the use of non-cryogenically produced nitrogen economically unattractive.
Japanese patent application number 62-144889 filed on Jun. 10, 1987 discloses a method of producing non-oxidizing and non-decarburizing atmosphere in a continuous heat treating furnace operated under vacuum by introducing 1% or less hydrogen and low-purity nitrogen with purity 99.995% or less into the hot zone of the furnace through two separate pipes. The key feature of the disclosed process is the savings in the amount of nitrogen gas achieved by increasing the operating pressure form 40 mm Hg to 100-150 mm Hg. This patent application does not set forth any information relating to the quality of the parts produced by using low-purity nitrogen in the furnace nor is there any disclosure in regard to the applicability of such a method to continuous furnaces operated at atmospheric to slightly above atmospheric pressures.
An atmosphere suitable for heat treating copper in a continuous furnace has been claimed to be produced by using a mixture of non-cryogenically produced nitrogen with hydrogen in a paper titled, "A Cost Effective Nitrogen-Based Atmosphere for Copper Annealing", published in Heat Treatment of Metals, pages 93-97, April 1990 (P. F. Stratton). This paper describes that a heat treated copper product was slightly discolored when all the gaseous feed containing a mixture of hydrogen and non-cryogenically produced nitrogen with residual oxygen was introduced into the hot zone of the continuous furnace using an open feed tube, indicating that annealing of copper is not feasible using an atmosphere generated by using exclusively non-cryogenically produced nitrogen mixed with hydrogen inside the furnace. Although there is no explicit mention about residual oxygen in the furnace, the reported experimental results do suggest incomplete conversion of residual oxygen in the furnace to moisture. At best the prior work suggests using atmosphere generated by prereacting residual oxygen present in the non-cryogenically produced nitrogen with a small amount of hydrogen in an external unit for heat treating copper.
Based upon the above discussion, it is clear that there is a need to develop a process for generating low-cost atmospheres inside continuous heat treating furnaces suitable for annealing and heat treating ferrous and non-ferrous metals and alloys using non-cryogenically produced nitrogen and a reducing gas such as hydrogen, a hydrocarbon, or a mixture thereof.